Conventional flip-chip bonding is to dispose a plurality of solder balls on the active surface of a chip as external electrical terminals. Then, solder balls are mechanically and electrically connected to the corresponding connecting pads of the substrate through flipping the chip to make the active surface face downward to the substrate followed by a reflowing process. However, the sidewalls of solder balls are arc so that when solder ball pitches become smaller and smaller, the adjacent solder balls are easily bridging to each other leading to electrical short so that flip-chip bonding using solder balls can not meet the requirements of fine-pitch applications such as ball pitch less than 100 um.
MPS-C2 (Metal Post Solder-Chip Connection) packaging includes an advanced flip-chip bonding technology by utilizing metal pillars with solder paste to replace solder balls. Saitoh et al. taught a bump structure for MPS-C2 package in U.S. Pat. No. 6,229,220 B1, it has developed metal pillars to replace solder balls as bump interconnection for flip-chip bonding by using solder paste to mechanically and electrically connect the metal pillars to the corresponding connecting pads of a substrate where the reflow temperature only can melt solder paste without reaching the melting point of metal pillars to keep their pillar's shapes. The pitch between metal pillars can further be shrunk without bridging issues as the conventional solder balls have.
As shown in FIG. 1, a conventional MPS-C2 semiconductor package 1 primarily comprises a substrate 10, a chip 20, and an encapsulant 30. A plurality of metal pillars 21 are disposed on the bonding pads 23 of the chip 20 where the metal pillars 21 are mechanically and electrically connected to a plurality of corresponding connecting pads 12 on the substrate 10 by solder paste 22. Moreover, the encapsulant 30 is underfilling material having good fluid properties to fill into the flip-chip gap between the chip 20 and the substrate 10. Since the CTEs between the chip 20 and the substrate 10 are different plus the curing shrinkage of the encapsulant 30, the semiconductor package 1 is vulnerable for package warpage. Furthermore, as the pitches between metal pillars 21 become smaller and smaller, the solder paste 22 of the adjacent metal pillars 21 would easily bridge with each other during a reflowing process leading to electrically short between the adjacent metal pillars 21.